FIELD OF INVENTION
The present invention relates to an improved Heat Exchanger to raise the degree of superheat of superheated cold reheat steam (CRH) from high pressure turbine of a fast breeder reactor by live steam from steam generator.
BACKGROUND OF THE INVENTION
Previously, a design does exist for an application which is closest to the application for which this proposed invention is created. The past design uses an arrangement where heating fluid is a saturated steam (not a superheated steam as in proposed invention), flowing though spiral tubes from a common header and the fluid being heated which is a low pressure steam (not a superheated steam as in proposed invention) flows over the tubes in a straight path. The past design is such that tubes are angled towards a central header, to enable flow of any condensate formed inside the tubes, to the central header to be taken for further processing. The past design has housing having frustoconical portions on both ends with a central header also having similarly shaped ends. Past design has man-way accesses on both ends for maintenance and cleaning purposes. The disadvantages associated with past design are enlisted below:

3.1 Complicated design from manufacturing & maintenance point of view – spiral shaped tube heat exchangers are now outdated and very complex from manufacturing point of view. Besides, this design does not offer the possibility to replaced or plug any tube which has started leaking. There is no provision to remove the shell for carrying out any maintenance activities.
3.2 Not a compact design : The tubes are positioned helically around a central axis, with major expanse in horizontal direction (taking more area of ground).
3.3 The design is for saturated condition of steam : which is wet steam.
3.4 Past invention does not reflect on occurrence of phenomenon of flow-induced vibrations or its resistance to flow-induced vibrations.
OBJECT OF THE INVENTION
Therefore, it is an object of the invention to propose an improved Heat Exchanger/live steam reheater to raise the degree of superheat of superheated cold reheat steam (CRH) from high pressure turbine of a fast breeder reactor by live steam from steam generator which is capable of making maximum heat transfer to raise the superheat temperature of a steam already in superheated state.

Another object of the invention is to propose an improved Heat Exchanger to raise the degree of superheat of superheated cold reheat steam (CRH) from high pressure turbine of a fast breeder reactor by live steam from steam generator which takes less ground area having major expanse in vertical direction.
A further object of the invention is to propose an improved Heat Exchanger to raise the degree of superheat of superheated cold reheat steam (CRH) from high pressure turbine of a fast breeder reactor by live steam from steam generator which can perform with minimal pressure drop (within 1.50 kg/sq.cm-shell side).
SUMMARY OF THE INVENTION
The design is suitable for application called live steam reheater whose function is to reheat expanded steam (hereinafter called as cold reheat steam or CRH) from high pressure turbine of a fast breeder reactor plant, to a higher temperature before being admitted to lower pressure turbine. The hot steam (the heated CRH) from live steam reheater is called hot reheat steam (or HRH). The cold fluid is a superheated steam and by getting heat from hot fluid, the degree of superheat or superheat temperature is further raised (from 267.30C to 340.30C). The hot fluid is live steam from steam generator having sufficiently high degree of superheat (temp.4900C). The heat

exchanger design is such that hot fluid i.e., live steam flows through tubes and cold fluid i.e, CRH flows over tubes through a shell that encloses the bundle of tubes.
The design objectives translated into a design with a unique combination of following features:
5.1 Use of Alloy Steel Low fin U-tubes.
5.2 Use of a Alloy steel Partitioned hemispherical header for tube side flow.
5.3 Use of a specially designed swing-like impingement plate.
5.4 Use of Self-reinforced process nozzles.
5.5 Use of Single Segmental NTIW baffles.
5.6 Use of Deresonating Baffles.
5.7 Use of larger flow sections (a non-standard baffle cut of 28.5% and a tube pitch ratio of 1.70).
5.8 Use of Rotated square tube pitch.
A heat exchanger/Live Steam reheater (H) (Fig. 1) specifically designed to raise the degree of superheat i.e., increase the superheat temperature of a steam already in superheated state extracting heat from another superheated steam to the extent it is

condensed, comprising of a bundle of low-finned U-tubes (1, Fig.1) of low alloy steel material positioned vertically in a such a way that U-bends point upwards, encased in a cylindrical shell (2, Fig.1) carrying self-compensating nozzles (7,9-Fig.1) at topmost and bottommost points of shell providing inlet and outlet of shell side fluid and having a specially designed swing-like impingement plate (13, Fig.1) to protect tubes from damaging impact of incoming steam , with U-tubes being supported by segmented baffles (12, Fig.1) having a baffle cut percentage of 28.5% and having no tubes in window (all tubes having same span between supports), with de-resonating baffles (Fig.4) in every span, with U-tubes placed at a pattern of rotated square (Fig. 2) with each other and attached to a tub-shaped tubesheet (6, Fig.1) made of low-alloy steel, with tubesheet having inner surface profile designed in such a way that smoothly interfaces with inner profile of hemispherical head (3, Fig.1) that is a housing for channeling tubeside fluid inlet and outlet, with hemispherical head having two self-compensating nozzles (10,11-Fig. 1) providing inlet and outlet of tubeside fluid, a specially shaped partition plate (4, Fig.1) to separate inlet and outlet chambers of same hemisphere, with hemispherical head having a man-way access (5, Fig.1) that is closed by a self-sealing cover (14, Fig.1), whereby heat is exchanged between hot fluid flowing through tubes and cold fluid flowing over tubes with minimal flow-induced mechanical & acoustic vibrations and minimal pressure drop.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig. 1: Shows the invented Heat exchanger.
Fig. 2: Shows 45 degree tube layout.
Fig. 3: Shows no tubes in window baffle (NTIW).
Fig. 4: Shows de-resonating baffles used to keep acoustic vibration in limits.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION
Aim of invention is to provide a compactly designed heat exchanger suitable for a compact plant layout (Refer Figure 1). Therefore, a configuration with U-tubes (1) suitable for vertical installation was selected. A U-tube provides a compact two-pass design while allowing for differential thermal expansion of its two legs subjected to different metal temperatures. To have the most optimum mechanical design the fluid with higher pressure i.e., live steam or hot fluid (at pressure 170 kg/sq.cm (a)) was chosen to flow through tubes while the fluid with lower pressure (33.29 kg/sq.cm (a)) was chosen to flow through shell (2). The hot fluid flowing through U-tubes releases heat in two stages- first desuperheats and then condenses, in the same U-tube. The design is such that there is no condensation in first leg of the tube so that no condensate falls back and interferes with the incoming steam. U-tubed heat exchanger

would have both inlet and outlet chambers housed in one channel with partition designed for differential pressures of inlet & outlet flow. Hemispherical type heads for the geometry of channel are a good choice as they lead to smallest design thickness in high hydrostatic internal pressure cases. Hence, a hemispherical type channel (3) was selected with partitioninq (4) of inlet and outlet chambers in such a way that it provided man-way access (5) for maintenance purposes. The man-way access is closed by means of a self-sealing cover (14). A unique tub-shaped tubesheet (6) has been designed to bear all mechanical loadings, besides providing a leak-tight joint between tube side and shell side fluids. The inner surface profile of tubesheet is such that it smoothly interfaces with inner profile of hemispherical channel, thereby eliminating any stress concentration on account of sharp changes in geometry sections. The flow on shell side is such that CRH i.e., shell side fluid enters at the topmost point through CRH inlet (7) and leaves at the bottommost point of shell (2) through HRH Outlet (9) and receives heat from tube side fluid by flowing transverse to axis of tubes in tube length sections created by segmental baffles (12). The hot fluid that enters as live steam through live steam inlet (10) provided on hemispherical channel (3) enters first legs of U-tubes (1) and leaves as condensate from second legs of the U-tubes (1) and exits the hemispherical channel (3) through condensate outlet (11).The heavy metal removal on shell as well as hemispherical channel for creating such large sized inlet/outlet nozzle (7,9,10,11) holes (sizes 700 NB, 800NB, 200NB & 300NB) requires compensation which

is normally provided by compensating pads which have diameters double the diameter of nozzle being compensated. However, space limitation called for use of self-compensating nozzles designed to carry all the reinforcement required. Heavy steam flow through CRH inlet impinging the U-tubes (1) at U-bends could create damaging effect, however, the U-tube geometry would not allow proper attachment of impingement plate owing to its shape therefore a specially shaped impingement plate (13) attached to shell, supported by a swing-like structure attached to dished end (8), has been designed to protect U-tubes (1) from impingement effect.
Another aim of invention is to provide a heat exchanger with minimal pressure drop (within 1.50 kg/sq.cm-shell side). To minimize pressure drop especially on shell side where flow (and thus, velocity) is very high of CRH steam, following design steps were taken :
(i) Selection of rotated square pitch (450) led to improved results in form of reduced pressure drops as compared to triangular pitches. (Figure-2)
(ii) Optimized the Baffle cut to a new level (28.5%) that reduces the pressure drop due to sudden changes in flow direction.
(iii) Increasing the Baffle pitch, although compromising on heat transfer coefficient & increasing the concerns related to flow-induced vibrations, helped in reducing pressure drop significantly.

However, above steps taken to minimize pressure drop on shell side led to following undesirable effects:
(a) Deteriorated heat transfer coefficients on account of increased flow sections in
form of increased baffle pitches (or unsupported spans), rotated square pitches, large
baffle cuts.
(b) Unacceptable levels of damaging flow-induced tube vibrations & acoustic vibrations
on account of large unsupported spans and large sized shell.
Following design improvements were made to minimize above detrimental effects:
Selection of Finned tubes for improved heat transfer coefficient
Tighter tolerance on allowable pressure drop on shell side necessitated use of larger unsupported spans between baffles. This seriously affected Heat Transfer Coefficient, while already the fluids involved are vapours i.e., have inherently low heat transfer coefficients. To counter this problem, finned tubes were used to aid in improving heat transfer coefficient.
Flow-induced Vibration Control
A detailed analysis of flow-induced vibrations was carried out. Iterative approach was adopted in varying parameters like tube pitch type, ratio of tube pitch-tube OD,

type of baffle configurations, unsupported spans etc. Following choices emerged which led to betterment of flow-induced vibration analysis results:
(i) Selection of 450 tube layout - Good results were obtained using 450 tube layout as this, along with a larger pitch-tube OD ratio, helps in reducing the lift coefficient which is one of the factors increasing the Vortex Shedding amplitude. Larger amplitudes could lead to tube damages. (For layout see Fig. 2)
(ii) Selection of large Pitch-to-tube dia. ratio - Although a large tube pitch means compromise in heat transfer coefficient but this helps reduce cross-flow velocity past the tube and also reduce the pressure drop. If cross-flow velocity exceeds critical velocity, exceptionally high amplitudes of tube are resulted which could further lead to a seriously damaging phenomenon called fluid-elastic instability.
(iii) Selection of NTIW baffles - No-tubes-in-window (NTIW) type baffles (12) were selected. This offered advantages like all tubes in a pitch had same span unlike single segmental baffles where the tubes in window area have double the unsupported span. This provided flexibility in choosing larger spans. However, eliminating tubes from the windows usually required a larger diameter shell to provide sufficient heat transfer surface. A further consequence of NTIW bundles was that pressure drop expended from one cross-pass to the next would be ineffective because no heat would be transferred in windows. (See Fig. 3).

(iv) Protection of impingement on U-tube Span - Increased the shell length beyond the top of U-bends to lead to a hollow portion and placed the shell steam inlet in such a way that only a small portion of U-bends fell under the projected area of nozzle. Further a special impingement plate (13 of Figure 1), taking a swing-like support from shell was designed to avoid direct hitting of U-bend portion under the nozzle.
(v) Addition of De-resonating baffles - Use of 450 tube layout is known to add to acoustic problems. Further larger shell diameters reduce the acoustic frequency. To keep acoustic vibrations in limits de-resonating baffles were used in each baffle pitch, perpendicular to tube axis, at a location in tube layout which allows breaking of first five modes of acoustic waves at places away from wave nodes. Acoustic frequency is inversely proportional to length between reflecting valves. Hence, these baffle help in increasing acoustic frequency significantly.

WE CLAIM
1. An improved Heat Exchanger/live steam reheater (H) to raise the degree of
superheat of superheated cold reheat steam (CRH) from high pressure turbine of a fast breeder reactor by live steam from steam generator, the said reheater comprising;
a bundle of finned U tubes (1) disposed in a cylindrical shell (2) for improving heat transfer coefficient with U-bends of the said U-tube pointing upwards;
self-compensating nozzles (7, 9) disposed for allowing shell side fluid (cold reheated steam) from high pressure turbine to enter at topmost point of the said shell through Cold reheat steam inlet (7) and to leave at the bottom most point of the said shell (2) through Hot reheat steam outlet (9) for admission into a lower pressure turbine;
a plurality of No-tubes-in-window (NTIW) segmental baffles (12) disposed for making flow of fluid transverse to axis of tubes to receive heat from tube side fluid;
a hemispherical type channel (3) disposed along with a partition (4) for an inlet and an outlet chambers creating a man-way access (5) for maintenance work;
a self sealing cover (14) for closing the said access (5);
an impingement plate (13) attached to shell (2) supported by a swing-like structure (15) attached to dished end (8) disposed for protecting U-tubes (1) from impingement effect;

characterized in that;
the hot fluid that enters as live steam through live steam inlet (10) provided on hemispherical channel (3) enters first legs of U-tubes (1) and leaves as condensate from second legs of U-tubes (1) and exits the hemispherical channel (3) through condensate outlet, wherein the tubes are arranged in a 45 degree square pitch layout for arresting pressure drops, when de-resonating baffles are disposed for increasing acoustic frequency wherein a tub-shaped tube sheet (6) smoothly interfaces with inner profile of hemispherical head (3) when heat is exchanged between hot fluid flowing through tubes (1) and cold fluid flowing over tubes (1) with minimal flow-induced mechanical and acoustic vibrations and minimal pressure drop.
2. An improved heat exchanger as claimed in claim 1, wherein segmented baffles (12) supporting the U-tubes (1) has a baffle cut percentage of 28.5% with all tubes having same span between supports.
3. An improved heat exchanger as claimed in claim 1, wherein the tub-shaped tube sheet (6) is made of low-alloy steel.
4. An improved heat exchanger as claimed in claim 1, wherein the U-tubes (1) are made of low-alloy steel.